This invention is an improvement in measuring male and female tapered or conical parts. More particularly, this invention is an apparatus and method for measuring such parts that use non-contact linear measurement technology and that can accurately and repeatably measure tapered or conical parts. The inventive apparatus and method are particularly suited to measuring tapered parts that exhibit large deviations from their desired measurements.
The 25th Edition of Machinery""s Handbook, Industrial Press Inc., New York, 1996, at pp. 689-693, describes measurement of a tapered or conical part. This handbook describes suitable methods for determining measurements of an exteriorly tapered or conical (male) part and for determining measurements of an interiorly tapered or conical (female) part.
A method for determining the measurement of an exteriorly tapered or conical (male) part is as follows. The axis of a vee-block aligns with the sides of a sine-bar, as in FIGS. 1A-C. The male tapered part 10 to be measured positions in the vee-block 12. The male tapered part 10 aligns perpendicularly to the transverse axis of the sine-bar 14. The sine-bar 14 offsets to the proper height to define a flat plane for the top surface 16 of the male tapered part 10, as shown in FIG. 1C. The sine-bar 14 is set to the angle B=(C+A/2), where A/2 is one-half the included exterior angle of the male tapered part 10, as shown in FIG. 1C. Where D is the included angle of the precision vee-block 12, as shown in FIG. 1B, the angle C, shown in FIG. 1C, is calculated from the formula:       Sin    ⁢          xe2x80x83        ⁢    C    =            Sin      ⁢              xe2x80x83            ⁢              A        /        2                    Sin      ⁢              xe2x80x83            ⁢              D        /        2            
This determines the total included angle of the male tapered part 10. If all points along the top surface 16 of the tapered part 10 are co-linear and co-planar, as in FIG. 1C, this confirms the planarity (absence of concavity/convexity) of the exterior surface of the male tapered part 10 along that line. By continuing such replicate measurements along the surface of the male tapered part 10, the concavity/convexity for the entire part 10 is determined.
Machinery""s Handbook also describes measurement of an interiorly tapered or conical (female) part 18, with reference to FIG. 2. The method uses individual balls B, B1 of two or more different sizes that each has a given fit diameter within the interior of the female tapered part 18. The principle of the ball method of taper measurement is described with reference to the gauge 20 in FIG. 2. When two balls B, B1 of unequal diameters are either in contact or a known distance apart, lines tangent to both their circumferences will represent an angle or taper. The degree of the angle or taper depends upon the diameters of the two balls B, B1 and the distance C between their centers. The gauge 24 in FIG. 2 consists of two adjustable straight edges A, A1, in contact with balls B, B1. The angle xcex1, or the taper between the straight edges A, A1, depends upon the diameters of the balls B, B1 and the center distance C between the balls. By measuring these three dimensions accurately, the gauge 20 can be set to a given angle.
The first ball B inserts into the taper defined between A and A1 and the height of B from the base of the taper is measured. Then a second, larger ball B1 inserts into the taper and the difference in heights between B and B1 from the base of the taper is measured. The difference between the larger and smaller diameters of the female tapered part 18 (i. e., diameters of the balls B and B1) and the length of the longitudinal axis of the female tapered part 18 (i. e., center distance C) define the planarity (absence of concavity/convexity) of the interior surface of the female tapered part 18.
All state-of-the-art-measuring devices utilize some variation on the principles of measurement of the interior (female) or exterior (male) conical or tapered part as described above. For example, current commercially available gauges using air jet nozzles employ the following method for measuring exterior (male) tapers.
A typical conventional gauge uses a funnel-shaped receptacle, known as a xe2x80x9cpot.xe2x80x9d The interior pot surface has orifices for small air jet nozzles that direct airflow at the interior of the pot and at an exteriorly tapered part supported within the pot. The air jet nozzles arrange in horizontally aligned pairs, and the nozzles of each pair position 180xc2x0 apart from each other. The gauge generally has three pairs of air jet nozzles arranged at three different horizontal levels, respectively, within the pot. A first set of nozzles positions to measure a larger diameter of the taper. A second set of nozzles positions at about the middle of the taper height to measure an intermediate diameter of the taper. A third set of nozzles positions to measure a smaller diameter of the taper.
The interior taper of the pot provides contact support for the male tapered part to be measured. The air pressure exerted by each aligned pair of air jet nozzles within the pot is measured with the exteriorly tapered part in place and with an exteriorly tapered mastering part in place. When the mastering part is in place, a minimal air gap exists between the mastering part and the pot. With the mastering part in place, an initial pressure is measured to calibrate the gauge. The air gap between the mastering part and the pot is determined at each set of air jet nozzles. Then, the mastering part is removed and replaced with a male tapered part to be measured.
There is a difference between the pressure measurement with the male tapered part to be measured in the pot and the pressure measurement with the male tapered mastering part in place. The difference in air pressures determined by each pair of air jet nozzles is used to calculate the difference between the diameter of the mastering part and the diameter of the tapered part horizontal to that pair of air jet nozzles. The air gap between the male tapered part being measured and the pot is determined and compared to the air gap determined between the mastering part and the pot at each set of air jet nozzles. Diameters are thus measured at three different locations along the height of the tapered part. The distance between any two diameters along a vertical axis of the tapered part is known. Lines tangent to the circumferences of both of the circles defined by these diameters will form the angle of the tapered part. This calculation may be made by hand or by the computer measuring the pressure differences.
The top and bottom diameters, measured by the top and bottom air jet nozzles, establish a diameter difference (similar to that determined by the xe2x80x9cball dropxe2x80x9d described in Machinery""s Handbook). The third diameter, measured by the third set of air jet nozzles, determines the straightness of the taper (its concavity/convexity), by indicating whether the three points are co-linear. Points are co-planar by the construction of the pot.
The diameters are measured with the tapered part resting in contact with the pot in a xe2x80x9cgravity seat;xe2x80x9d that is, the part rests there under its own weight. The tapered part positions in the pot relative to the air jet nozzles. If the tapered part should fit off-center and over on one side of the pot, one of the air jet nozzles will be plugged and restricted. The restricted air jet nozzle will register an incorrect higher-than-average pressure, while the corresponding horizontally opposite air jet nozzle will register an incorrect lower-than-average pressure. The pair of air jet nozzles operatively connects, so that the corresponding gaps between the tapered part and each air jet nozzle are averaged and the diameter of the tapered part at that point can be measured.
The currently available system of measuring tapered parts is limited, because the air jet nozzles must position close to the surface of the tapered part (i.e., with only a small gap between the tapered part to be measured and the pot). The tapered part may tend to position off-center, or may tend to deviate from the expected mean size, so that a surface of the tapered part is too far from the air jet nozzle. When this happens, the tapered part is not within the linear measuring range of that air jet nozzle and a correct diameter measurement cannot be obtained. Close positioning of the tapered part to the pot and to the air jet nozzles is required so that the diameters of the tapered part position within the linear measurement range of the jets to obtain reliably repeatable measurements. If the gap between the tapered part and the pot becomes too large, the pressure drop between the air pressure measurement within the empty pot and the air pressure measurement with the tapered part inserted will not be linear. In this case, exact measurement can only be obtained if the tapered part is very close to its required finished dimensions. Measurement of tapered parts that deviate greatly from the required finished dimensions will be inaccurate and will not give reliably repeatable measurements.
If the tapered part seats within the pot with the longitudinal axis of the tapered part offset from true vertical alignment, the air jet nozzles will be unable to accurately measure a true diameter of the part. The air jet nozzles will measure some diameter of an elliptical portion of the tapered part and incorrectly report it as a diameter of a circle.
An apparatus for measuring a freely suspending tapered part according to this invention has a plurality of non-contact linear measuring probes positioned to determine critical measurements on the tapered part. The tapered part, which is symmetrical about its central axis, suspends in a position normal to its axis. The tapered part may be symmetrical about its central axis in two dimensions, such as a wedge (male taper) or a trough (female taper), or in three dimensions, such as a cone (male taper) or a funnel (female taper).
A first pair of non-contact linear measuring probes aligns in non-contact position to a larger portion of the part. The part suspends so that the larger portion is within the linear measuring range of the first pair of probes. The first pair of probes measures a diameter of the part at the larger portion. A second pair of non-contact linear measuring probes aligns in non- contact position to a smaller portion of the part. The part suspends so that the smaller portion is within non-contact linear measuring range of the second pair of probes. The second pair of probes measures a diameter of the part at the smaller portion. By measuring the diameters of the part at the larger and the smaller portions and the known vertical distance between the diameters, the included angle of the part is calculated.
A fifth non-contact linear measuring probe may position in vertical alignment with one each of the first and the second pairs of probes. This fifth probe will align in a non-contact linear measuring position to the part. The three points, that is, the first point (of one of the first pair of probes), the second point (of one of the second pair of probes), and the fifth point (vertically aligned with the first and second points), align vertically with the vertical axis of the part. By measuring the straightness of the line defined by these three points, the concavity/convexity of the part surface is also determined.
The probes may be based on any type of non-contact measuring technology capable of non-contact linear measurement. Suitable examples include a capacitance probe, a laser micrometer, a light or sound based non-contact linear measuring device, or an LVDT (linear voltage displacement transducer) with a powered probe that activates once the taper is positioned. Non-contact linear measurement systems are preferable to contact linear measurement systems, because contact linear measurement will show hysteresis in the measurement.
The tapered part can be a male tapered part, in which case the probes are retained within a female tapered support. The male part is freely suspending. For example, the male part may suspend from the support, for example, by a rim horizontally extending from an upper surface of the male tapered part. The male tapered part can be a cone or a wedge. The tapered part can be a female tapered part, in which case the probes are retained within a male tapered support and the female part is freely suspending. The female tapered part can be a funnel or a trough.
This invention also comprises a method of measuring a tapered part. A tapered part to be measured suspends in a position normal to a central axis of the part. A first pair of non-contact linear measuring probes aligns in non-contact position to a larger portion of the part and at a larger portion of the support. The part is positioned with the larger portion within non-contact linear measuring range of the first pair of probes, so that the first pair of probes can measure the diameter of the part at the larger portion.
A second pair of non-contact linear measuring probes aligns in non-contact position to a smaller portion of the part and at a smaller portion of the support. The part is positioned with the smaller portion within non-contact linear measuring range of the second pair of probes, so that the second pair of probes can measure the diameter of the part at the smaller portion.
The first pair of probes measures the larger diameter of the empty support. The second pair of probes measures the smaller diameter of the empty support. The tapered part freely suspends in a position normal to a central axis of the part and within non-contact linear measuring range of the probes. The first pair of probes measures the gap between the perimeter of the diameter of the larger portion of the part and the perimeter of the aligned larger diameter of the support. The second pair of probes measures the gap between the perimeter of the diameter of the smaller portion of the part and the perimeter of the aligned smaller diameter of the support. By measuring the diameters of the part at the larger and the smaller portions and the vertical distance between the midpoints of the diameters, the included angle of the part is measured.
A fifth non-contact linear measuring probe may further align in a non-contact position to a fifth point on the part vertically different from the larger and smaller portions. The part is positioned with the fifth point within non-contact linear measuring range of the fifth probe, to measure the planarity of the part surface (convexity/concavity) in conjunction with one each of the first and second pairs of probes. The fifth probe vertically aligns with one each of the first and second pairs of probes. A first point on the larger diameter circumference, a second point on the smaller diameter circumference and a fifth point determined by the fifth probe determine a line in vertical alignment with a vertical axis of the part. The planarity of the line determined by the first and second points and the fifth point determines the concavity/convexity of the part.
The probes may be any non-contact linear measuring probes, including a capacitance probe, a laser micrometer, or any other suitable non-contact linear measuring technology. The support may be a funnel, a trough, a cone or a wedge and the part may correspondingly be a cone, a wedge, a funnel or a trough.